Valve metal powder and solid electrolytic capacitor using same

ABSTRACT

A valve metal powder with optimized particle size distribution, and a solid electrolytic capacitor which uses this is provided. A valve metal powder for a solid electrolytic capacitor is used which is an agglomerate powder to be used for manufacturing an anode with a structure in which valve metal powder is formed in a layer on a valve metal base material, and contains at least 90% of all the powder within a particle size range of 1 μm to 50 μm. It is desirable that at least 90% of all the powder is contained within a particle size range of 1 μm to 30 μm. Moreover, preferably the product of BET specific surface area and specific gravity (d 25 ) is greater than 17 m 2 /g.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid electrolytic capacitor whichuses an anode made by integrating metal powder and a lead sectionthrough sintering with a structure in which valve metal powder isaccumulated in a layer on a valve metal base material, and a valve metalpowder used for this.

2. Description of the Related Art

In the case of molding valve metal powder in a layer on a valve metalbase material, for example, as described in Japanese Unexamined PatentPublication No. 2003-209028, by manufacturing a dispersion liquid(paste) using valve metal powder, and molding it through an industrialmethod such as metallic mask printing, and sintering it in vacuum, thevalve metal base material and the layered powder molding part areintegrated, and it is possible to use it as an anode for a solidelectrolytic capacitor.

However, conventionally, with a type of capacitor using valve metalpowder as its anode, such as a tantalum capacitor, the valve metalpowder is provisionally molded through an industrial method called drypress, and an anode is then made by sintering. At this time, agglomeratepowder of the valve metal powder is used. However, the particle sizedistribution of the agglomerate powder has a central particle size ofaround 100 μm and the distribution range is from around 10 μm to 200 μm.

If a powder layer is formed by dispersing such an agglomerate powderused in the dry press method, over the aforementioned paste, sufficientwelding between the valve metal base material and the agglomerate powderdoes not occur at the time of sintering, and it becomes prone toflaking. Furthermore, voids (hole-like cavities) are likely to occur inthe layer of the powder molding part, and the occurrence of such voidsnot only decreases the product yield, but also has been the cause ofdegradation in the properties of solid electrolytic capacitors.

The occurrence of the voids will be described with reference to thedrawings. FIG. 2 is an explanatory drawing illustrating an anode for asolid electrolytic capacitor made using a powder for dry press.

When the valve metal powder is agglomerated, a method is used where thepowder (primary particles) is weakly bonded through heating in order toobtain an agglomerate powder (secondary particles). However, in the caseof large agglomerate powders such as the agglomerate powder for the drypress method where the central particle size is adjusted to around 100μm, sintering of the powder is already degraded by heating in theagglomeration process. Therefore, there is a problem that, as describedbefore, sufficient welding does not occur between the valve metal basematerial and the agglomerate powder at the time of sintering.

Moreover, since the voids originate from the gaps between theagglomerate powder particles, then in the case of using largeagglomerate powder, the gaps become large and voids are likely to occur.

Cracking and warping due to sintering shrinkage will be described withreference to the drawings. FIG. 3 is an explanatory drawing illustratingan anode for a solid electrolytic capacitor made using non-agglomeratepowder.

In the case of using non-agglomerate powder with a central particle sizeof around 0.3 μm and a distribution range of around 0.2 μm to 1 μm, itdoes not undergo a heating agglomeration process. Therefore, the degreeof sintering is very high, and the welding with the valve metal basematerial can be sufficient. However, deformation due to shrinkage duringthe sintering becomes large, and cracking of the powder layer occurs, orwarping of valve metal base material on which the powder layer is weldedoccurs. Therefore, the properties and yield of the capacitor are againdegraded.

SUMMARY OF THE INVENTION

As described before, regarding the manufacture of an anode for a solidelectrolytic capacitor with a structure in which valve metal powder isaccumulated in a layer on a valve metal base material, the particle sizedistribution of the valve metal powder has a large effect on theproperties and yield of the capacitor. Therefore, an object of thepresent invention is to provide a valve metal powder with optimizedparticle size distribution, and a solid electrolytic capacitor whichuses this.

The valve metal powder for the solid electrolytic capacitor according tothe present invention is an agglomerate powder to be used formanufacturing an anode with a structure in which valve metal powder ismolded in a layer on a valve metal base material, and contains at least90% of all the powder within a particle size range of 1 μm to 50 μm.

Also, it is desirable that at least 90% of all the powder is containedwithin a particle size range of 1 μm to 30 μm.

Moreover, preferably a product of BET specific surface area and specificgravity (d²⁵) is greater than 17 m²/g.

Furthermore, it is desirable that the aforementioned valve metal istantalum, tantalum alloy, niobium, or niobium alloy.

The solid electrolytic capacitor according to the present invention ismade using a valve metal powder for a solid electrolytic capacitoraccording to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing an anode for a solidelectrolytic capacitor according to the present invention.

FIG. 2 is an explanatory drawing showing an anode for a solidelectrolytic capacitor made using a dry press powder.

FIG. 3 is an explanatory drawing showing an anode for a solidelectrolytic capacitor made using a non-agglomerate powder.

FIG. 4 is an external appearance photograph of an anode for a solidelectrolytic capacitor made in a first example of the present invention.

FIG. 5 is an external appearance photograph of an anode for a solidelectrolytic capacitor made in a first comparative example.

FIG. 6 is an external appearance photograph of an anode for a solidelectrolytic capacitor made in a third comparative example.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is an explanatory drawing illustrating an anode to be used for asolid electrolytic capacitor according to the present invention.

Regarding the manufacture of the anode for a solid electrolyticcapacitor with a structure in which a valve metal powder is accumulatedin a layer on a valve metal base material, prior to the formation of thepowder layer using a valve metal powder dispersion liquid by a processsuch as metallic mask printing, granulation and calcination areperformed for the powder used in the valve metal powder dispersionliquid, using an appropriate method and conditions so as to produceagglomerate powder in which particle size distribution is restrictedwithin a constant range. As a result, control of cracking, warping,voids, and flaking of the powder layer that occur during the sinteringand molding of the anode becomes possible, so that it is possible toimprove the properties and yield of the solid electrolytic capacitor.

The valve metal powder for a solid electrolytic capacitor according tothe present invention is an agglomerate powder to be used formanufacturing an anode with a structure in which valve metal powder ismolded in a layer on a valve metal base material, and contains at least90% of all the powder within a particle size range of 1 μm to 50 μm.

Furthermore, it is desirable that aforementioned valve metal istantalum, tantalum alloy, niobium, or niobium alloy. The valve metal tobe used is selected in accordance with the required capacitycharacteristics, costs and so forth.

As a method for obtaining valve metal agglomerate powder, firstly thereis a method of manufacturing agglomerate powder from primary powder of avalve metal which has been subjected to reduction beforehand; involvingfor example, reducing potassium tantalate fluoride using metallic sodiumto make metallic tantalum primary particles, and after this, calciningthe obtained metallic tantalum primary particles in a vacuum or inertgas atmosphere to obtain a metallic tantalum mass, and then crushingthis to obtain agglomerate powder. Secondly, there is a method ofmanufacturing agglomerate powder of a valve metal by initiallyconverting into agglomerate powder in an oxide form and reducing it;involving for example, granulating tantalum pentoxide primary particles,and baking in the atmosphere to convert into tantalum pentoxideagglomerate powder, and then reducing using metallic magnesium to obtainvalve metal agglomerate powder.

Whether to granulate the valve metal primary particles as described inthe first method, or to granulate the primary particles of the valvemetallic oxide as described in the second method depends on the particlesize of the base powder as well as its particle size distribution, and aproper method is to be selected and used.

As a granulation method, there is a dry granulation method and a wetgranulation method.

Regarding the dry granulation method, there is: mesh granulation inwhich crushing is performed after agglomerating the powder and it isthen sieved; agitated mixing granulation in which granulation isperformed by agitatedly mixing the powder with an agitation blade whileadding a binder; and rolling and fluidizing dry granulation in whichgranulation is performed by agitatedly mixing the powder usingcompressed air while adding a binder. Regarding the wet granulationmethod, there are methods such as: fluidized bed granulation in whichgranulation is performed by repeatedly spraying, drying and coating asuspension consisting of fine powder of niobium pentoxide and purewater; and spray dry granulation in which granulation is performed bysimilarly spraying and drying the suspension.

The dispersion liquid is made by including at least a solvent and asolvent-soluble binder into the valve metal agglomerate powder, which isobtained in the abovementioned manner where at least 90% of all thepowder is contained within the particle size range of 1 μm to 50 μm, andthe product of BET specific surface area and specific gravity (d₂₅) ispreferably greater than 17 m²/g.

Having over 90% of the powder within the particle size range of 1 μm to50 μm is for preventing deformation, cracking, and warping during thesintering for obtaining the anode for a solid electrolytic capacitor byintegrating the valve metal base material and a layer of the powdermolding part as mentioned later.

Using the obtained dispersion liquid, a powder layer is formed on avalve metal base material through an industrial method such as metallicmask printing. Furthermore, after removing the solvent by drying, theproduct is sintered in a vacuum, the binder is removed throughdecomposition, and the valve metal base material and a layer of thepowder molding part are integrated, and an anode for a solidelectrolytic capacitor is thus obtained.

With respect to the anode for a solid electrolytic capacitor obtained inthe above manner, using a method called anode oxidation in which avoltage is applied within an electrolytic solution, a chemicalconversion coating serving as a dielectric is formed on the surface ofthe anode. Next a semi-conductor layer known as a solid electrolyticlayer made of manganese dioxide, polypyrrole and so forth is formed onthe chemical conversion coating. Then a conductive layer called acathode layer made of graphite, silver and so forth is formed to obtainan element part of the solid electrolytic capacitor. On this elementpart, an external electrode terminal for mounting, and a resin jacketfor element protection is attached, and high temperature superimposedvoltage aging combined with initial defect removal is performed, toobtain a solid electrolytic capacitor according to the presentinvention.

Moreover, for the valve metal agglomerate powder to be used, it isdesirable that the product of the BET specific surface area and thespecific gravity (d²⁵), is greater than 17 m²/g.

If the product of the BET specific surface area and the specific gravity(d²⁵) is greater than 17 m²/g, the capacitance of the solid electrolyticcapacitor to be obtained will be sufficiently manifested during the use.Therefore this is desirable.

The anode for the solid electrolytic capacitor according to the presentinvention is made using the valve metal powder for a solid electrolyticcapacitor of the present invention.

The solid electrolytic capacitor of the present invention is made usingthe valve metal powder for a solid electrolytic capacitor of the presentinvention.

FIRST EXAMPLE

After charging 300 g of niobium pentoxide powder of specific surfacearea 3.2 m²/g, particle size distribution 0.3 μm to 5.0 μm, and D₅₀=0.9μm, and 400 g of pure water into a 2 liter ball mill made of zirconia,and then mixing and dispersing for 15 hours, a suspension of mixed anddispersed niobium pentoxide fine powder was obtained.

To the obtained suspension, PVA was added to make up 0.5 mass % insolid. Then spraying, drying and coating of the suspension was repeatedinside a fluidized bed granulation apparatus, and after drying theobtained powder at 80° C., it was baked at a temperature of 1250° C. tomake an agglomerate powder of niobium oxide powder. After reducing theobtained agglomerate powder using Mg vapor at a temperature of 1000° C.,it was acid cleaned with hydrochloric acid, and further mixed with andreduced by magnesium (Mg), acid cleaned and washed in water, after whichit was subjected to vacuum drying to obtain 100 g of agglomerate powderof niobium metal.

Characteristics such as the particle size distribution, of the obtainedagglomerate powder of the niobium metal are shown in Table 1 and Table2.

100 g of the agglomerate powder of niobium metal obtained in the abovemanner and 55 g of a mixed solvent of polyvinyl alcohol (Kuraray Co.Ltd., PVA205-C) 5% solution as a binder and methyl alcohol was blended,and then kneaded for two hours using a shaker, and a dispersion liquidthus obtained.

Next, using a print mask made by providing a 3.0×4.0 mm rectangularopening in a 200 μm thick plastic sheet, a niobium dispersion liquidlayer was formed on a 3.1 mm×4.0 mm×50 μm thick niobium foil serving asa base material. Then after removing the solvent and moisture by dryingat 105° C. for 10 minutes, baking at 100° C. was performed for 20minutes inside a vacuum of 1×10⁻⁵ Torr to generate sintering among theniobium powders as well as sintering between the niobium foil and theniobium powder. As a result, an anode for a solid electrolytic capacitorwhere the mean thickness of the powder layer was 150 μm (of this, 50 μmwas the thickness of the base material) was obtained.

An external appearance photograph of the surface of the obtained anodefor a solid electrolytic capacitor is shown in FIG. 4. This is an anodewithout cracks, voids and so forth.

With respect to the aforementioned anode for a solid electrolyticcapacitor, anodization was performed by using 0.1% phosphoric acidsolution and applying a voltage of 20V, to thereby form on the surface achemical conversion coating which serves as a dielectric. The anode inthis condition was soaked in a 10 mass % iron dodecylbenzensulfonatemethanol solution for 5 minutes and after taking it out, the solvent wasair dried, and it was then soaked for 5 minutes in a pyrrole monomerliquid, and after taking it out it was left for one hour after which itwas washed with methanol, to thereby form on the chemical conversioncoating a conductive polypyrrole layer serving as a solid electrolyte.On this, a graphite layer and a silver layer was formed in this order bya paste immersion coating method. Then after attaching an externalelectrode terminal, a resin jacket was applied, and after this, agingwas performed by applying a voltage of 6V for ten hours in an atmosphereof 85° C., to obtain a niobium solid electrolytic capacitor.

Characteristics and yield of the obtained niobium solid electrolyticcapacitor are shown in Table 3.

FIRST COMPARATIVE EXAMPLE

100 g of an agglomerate powder of the niobium metal was obtained in thesame way as for the first example, except that the PVA to be added tothe suspension was 3.0 mass %.

The particle size distribution of the obtained agglomerate powder ofniobium metal is shown in Table 1 and Table 2. Compared to theagglomerate powder of niobium metal obtained in the first example, boththe central particle size (D₅₀) and the distribution range were larger.

Furthermore, an anode for a solid electrolytic capacitor was obtained inthe same way as for the first example.

An external appearance photograph of the surface of the obtained anodefor a solid electrolytic capacitor is shown in FIG. 5. Unlike the firstexample, it is evident that there are many hole-like cavities calledvoids. Furthermore, though not obvious from the external appearancephotograph, when it was ground down, it was found that 30% of the wholespecimen clearly showed partial flaking between the base material andthe powder layer.

Using the obtained anode for a solid electrolytic capacitor, a niobiumsolid electrolytic capacitor was obtained in the same way as for thefirst example.

The characteristics and the yield of the obtained niobium solidelectrolytic capacitor are shown in Table 3.

Compared to the first example, the ESR (equivalent series resistance)and leakage current properties were inferior, and a large difference inthe yield was found. This is due to the fact that, because of the voidsand flaking in the anode for the solid electrolytic capacitor, then atfirst the leakage current is increased, and this in turn deterioratesthe solid electrolyte during aging of the product. TABLE 1 Particle sizeSpecific Apparent range D₅₀ surface area density (μm) (μm) (m²/g)(g/cm³) Working example 1 0.8-30 8.55 2.95 0.98 Comparative   10-35095.75 2.65 0.85 example 1

TABLE 2 1-50 μm particle size Specific distribution gravity Product (%)(d²⁵)^(*1)) (m²/g)^(*2)) Working example 1 98.5 8.55 25.3 Comparative26.5 95.75 22.7 example 1^(*1))Specific gravity (d²⁵) is ratio of mass to water at 25° C.^(*2))Product is product of specific surface area and specific gravity(d²⁵)

TABLE 3 Leakage Capacitance^(*1)) current^(*2)) ESR^(*3)) Yield (μF)(μA) (mΩ) (%) Working example 1 40-55 0.2-80  25-35  80 Comparative30-40 0.2-1000 30-200 10 example 1^(*1))Capacitance measurement conditions: 120 Hz, 1 Vrms, Bias voltage1.5 V^(*2))Leakage current measurement conditions: Rated voltage 4 V applied,1 min^(*3))ESR measurement conditions: 100 Hz, 1 Vrms, Bias voltage 1.5 V

SECOND EXAMPLE

A reduction was performed on potassium tantalate fluoride 400 g usingmetallic sodium at 850° C. Then after acid cleaning, and water washing,this was passed through a vacuum heat treatment in a vacuum of 1×10⁻⁵Torr at a temperature 1200° C. for 0.5 hour, to thereby obtain asintered mass. Crushing was performed on the sintered mass using ahammer-type granulator at a rotational speed of 8000 rpm. Then, this wassubjected to mixing with and reducing by magnesium (Mg), acid cleaning,water washing, and vacuum drying, to thereby obtain 10 g of agglomeratepowder of tantalum metal.

Characteristics such as the particle size distribution of the obtainedtantalum metal agglomerate powder are shown in Table 4 and Table 5.

Because of the change from niobium to tantalum, a temperature of 1300°C. was used during the sintering of the anode for a solid electrolyticcapacitor. Apart from this, an anode for a solid electrolytic capacitorwas obtained in the same way as for the first working example.

The appearance of the obtained anode for a solid electrolytic capacitorwas almost the same as for the first example.

Using the obtained anode for a solid electrolytic capacitor, a tantalumsolid electrolytic capacitor was obtained in the same way as for thefirst example.

Characteristics as well as yield of the obtained tantalum solidelectrolytic capacitor are shown in Table 6.

SECOND COMPARATIVE EXAMPLE

Apart from changing the rotational speed of the hammer-type granulatorto 3000 rpm, 100 g of tantalum metal agglomerate powder was obtained inthe same way as for the second example.

Characteristics such as particle size distribution of the obtainedtantalum metal agglomerate powder are shown in Table 4 and Table 5.Compared to the tantalum metal agglomerate powder obtained in the secondexample, both the central particle size (D₅₀) and the range weregreater.

Moreover, an anode for a solid electrolytic capacitor was obtained inthe same way as for the second example. Its appearance and the degree offlaking were almost the same as for the first comparative example.

Using the obtained anode for a solid electrolytic capacitor, a tantalumsolid electrolytic capacitor was obtained in the same way as for firstexample.

Characteristics and yield of the obtained tantalum solid electrolyticcapacitor are shown in Table 6.

When the capacitor properties and yields of the second example and thesecond comparative example are compared, then in the same way as forfirst example and the first comparative example, capacitor propertiesand yields were inferior to those of the comparative examples usingpowder of larger particle size. TABLE 4 Particle size Specific Apparentrange D₅₀ surface area density (μm) (μm) (m²/g) (g/cm³) Working example2 0.5-20  4.75 1.95 1.96 Comparative 0.5-250 105.25 1.45 1.72 example 2

TABLE 5 1-50 μm particle size Specific distribution gravity Product (%)(d²⁵)^(*1)) (m²/g)^(*2)) Working example 2 95.6 16.6 32.4 Comparative12.7 16.6 24.1 example 2^(*1))Specific gravity (d²⁵) is ratio of mass to water at 25° C.^(*2))Product is product of specific surface area and specific gravity(d²⁵)

TABLE 6 Leakage Capacitance^(*1)) current^(*2)) ESR^(*3)) Yield (μF)(μA) (mΩ) (%) Working example 2 58-66 0.3-40   25-40  88 Comparative40-50 1-800 30-500 20 example 2^(*1))Capacitance measurement conditions: 120 Hz, 1 Vrms, Bias voltage1.5 V^(*2))Leakage current measurement conditions: Rated voltage 4 V applied,1 min^(*3))ESR measurement conditions: 100 Hz, 1 Vrms, Bias voltage 1.5 V

THIRD EXAMPLE

After charging 300 g of niobium pentoxide powder of specific surfacearea 5.1 m²/g, particle size distribution 0.2 μm to 3.0 μm and D₅₀=0.6μm, and 400 g of pure water into a 2 liter ball mill made of zirconia,and then mixing and dispersing for 15 hours, a suspension was obtained.

Niobium oxide agglomerate powder was then made by drying the obtainedsuspension by spraying it into a spray drier at a disk rotational speedof 14000 rpm and a temperature 200° C., then baking the obtained powderat a temperature of 1200° C. to make an agglomerate powder of niobiumoxide powder. After reducing the obtained agglomerate powder using Mgvapor at a temperature of 1000° C., it was acid cleaned withhydrochloric acid, and further mixed with and reduced by magnesium (Mg),acid cleaned and washed in water, after which it was subjected to vacuumdrying to obtain 100 g of agglomerate powder of niobium metal.

Characteristics such as the particle size distribution of the obtainedniobium agglomerate powder are shown in Table 7 and Table 8.

Further, in the same way as for the first example, an anode for a solidelectrolytic capacitor was obtained. The appearance of the surface ofthe obtained anode for a solid electrolytic capacitor was almost thesame as for the first example.

Using the obtained anode for a solid electrolytic capacitor, a niobiumsolid electrolytic capacitor was obtained in the same way as for thefirst example.

Characteristics and yield of the obtained niobium solid electrolyticcapacitor are shown in Table 9. The properties and yield of the obtainedniobium solid electrolytic capacitor were almost the same as for thefirst example.

THIRD COMPARATIVE EXAMPLE

After performing Mg vapor reduction at a temperature of 1000° C. on 300g of non-agglomerated niobium pentoxide powder used in the thirdexample, the product was acid cleaned with hydrochloric acid.Furthermore, mixing with and reducing by magnesium (Mg), acid cleaning,water washing and vacuum drying were performed, and 100 g of niobiummetal agglomerate powder was obtained.

The particle size distributions of the obtained agglomerate powder ofniobium metal are shown in Table 7 and Table 8. The degree ofagglomeration was markedly weak, and the central particle size (D₅₀) aswell as the range showed a particle size distribution close to that of aprimary particle.

Furthermore, an anode for a solid electrolytic capacitor was obtained inthe same way as for the first example.

An external appearance photograph of the surface of the obtained anodefor a solid electrolytic capacitor is shown in FIG. 5. Large cracks hadoccurred over practically all of the obtained anode for a solidelectrolytic capacitor. This is due to the fact that, because the powderused in the present comparative example did not undergo a heatagglomeration process, the degree of sintering was very high, and thepowder layer had significantly shrunk during the anode sintering.

Using the obtained anode for a solid electrolytic capacitor, a niobiumsolid electrolytic capacitor was obtained in the same way as for thefirst example.

Characteristics and yield of the obtained niobium solid electrolyticcapacitor are shown in Table 9.

Referring to Table 9, the characteristics and yield of the thirdcomparative example were both inferior compared to the third example.TABLE 7 Particle size Specific Apparent range D₅₀ surface area density(μm) (μm) (m²/g) (g/cm³) Working example 3 0.6-25  6.05 2.10 0.88Comparative 0.2-4.0 1.55 3.25 1.26 example 3

TABLE 8 1-50 μm particle size Specific distribution gravity Product (%)(d²⁵)^(*1)) (m²/g)^(*2)) Working example 3 96.3 8.56 18.0 Comparative21.2 8.56 27.8 example 3^(*1))Specific gravity (d²⁵) is ratio of mass to water at 25° C.^(*2))Product is product of specific surface area and specific gravity(d²⁵)

TABLE 9 Leakage Capacitance^(*1)) current^(*2)) ESR^(*3)) Yield (μF)(μA) (mΩ) (%) Working example 3 29-37 0.5-90 23-35  83 Comparative 10-57  10-800 50-800 0 example 3^(*1))Capacitance measurement conditions: 120 Hz, 1 Vrms, Bias voltage1.5 V^(*2))Leakage current measurement conditions: Rated voltage 4 V applied,1 min^(*3))ESR measurement conditions: 100 Hz, 1 Vrms, Bias voltage 1.5 V

A comparison was carried out between; a situation (first example) inwhich optimization was performed regarding the particle sizedistribution of valve metal powder to be used for manufacturing theanode for a solid electrolytic capacitor of a structure with a valvemetal powder layer accumulated on a valve metal base material; and asituation (first comparative example) which uses a valve metal powderhaving a particle size distribution suited for the dry press method,which is a conventional manufacturing method for anodes on solidelectrolytic capacitors. Since the powder suited for the dry pressmethod requires high fluidity, the particle size becomes larger thanthat of the powder suited for dispersion liquid (hereunder, the powderwith particle size suited for the dry press method is referred to aspress powder, and the powder with a particle size suited for dispersionis referred to as dispersion liquid powder).

In the first comparative example, in which a dispersion liquid (paste)is made using a press powder, and a powder layer is formed on the valvemetal base material by an industrial method such as metal mask printing,at the time of sintering, sufficient welding does not occur between thevalve metal base material and the powder, and flaking is likely tooccur. Furthermore voids (hole-like cavities) are likely to form on thepowder layer after sintering. For these reasons, as shown in Table 3,the capacitor properties and product yield of the first comparativeexample are degraded.

Contrary to the first comparative example, in the first example in whichthe powder layer is formed on the valve metal base material by making adispersion liquid using the powder having a proper particle sizedistribution, there was no occurrence of the sintering-related problemsthat occurred in the first comparative example, and also regarding thecapacitor properties and product yield of the first example, as shown inTable 3, favorable results were shown.

These results indicate similar results as in the case of using niobiumas the valve metal as seen in the first example and the firstcomparative example, even in the case of using tantalum as the valvemetal as seen in the second example and the second comparative example.

The third comparative example is the case where non-agglomerate primaryparticle powder was used for the valve metal powder, as illustrated inthe particle size distributions in Table 7 and Table 8. In the thirdcomparative example using primary particles, while the welding betweenthe valve metal base material and the powder layer becomes sufficient,the deformation becomes large due to the shrinkage during the sintering,and this causes cracking of the powder layer and warping on the basematerial to which the powder layer is welded. For these reasons, asshown in Table 9, capacitor properties and product yield of the thirdcomparative example are degraded.

Contrary to the third comparative example, in the third example usingdispersion liquid powder sintered and agglomerated with proper particlesize distribution as shown in Table 7 and Table 8, there was nooccurrence of the sintering-related problems that occurred in the thirdcomparative example, and also regarding the capacitor properties andproduct yield of the third example, as shown in Table 9, favorableresults were shown.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

1. A valve metal powder for a solid electrolytic capacitor which is anagglomerate powder to be used for manufacturing an anode with astructure in which valve metal powder is formed in a layer on a valvemetal base material, and contains at least 90% of all the powder withina particle size range of 1 μm to 50 μm.
 2. A valve metal powder for asolid electrolytic capacitor according to claim 1, wherein a product ofBET specific surface area and specific gravity (d²⁵) is greater than 17m²/g.
 3. A valve metal powder for a solid electrolytic capacitoraccording to claim 1, wherein said valve metal is tantalum, tantalumalloy, niobium, or niobium alloy.
 4. A valve metal powder for a solidelectrolytic capacitor according to claim 1, wherein the powder isobtained by spraying and drying of a suspension of niobium pentoxidepowder mixed and dispersed in pure water.
 5. A valve metal powder for asolid electrolytic capacitor according to claim 1, wherein the powder isobtained by sintering in a vacuum and crushing of reduced potassiumtantalite fluoride.
 6. A solid electrolytic capacitor made using thevalve metal powder for a solid electrolytic capacitor according to claim1.